1. Field of the Invention
The present disclosure relates to titanium oxide containing borosilicate glasses, wherein during the production thereof no arsenic and antimony compounds have been used as refining agents. Furthermore, the present disclosure comprises an environmentally friendly refining method for borosilicate glasses based on the use of oxygen containing selenium compounds as refining agents.
The glasses of the present disclosure are free of disturbing shades of yellow or brown, which often occur with the production of titanium oxide containing glass compositions, when no arsenic and antimony compounds are used as refining agents.
The glasses according to the present disclosure are particularly suitable as raw material for the production of infrared (IR) light conductors, cover glasses for photo sensors, ultraviolet (UV) filter glasses or the like.
2. Description of the Related Art
Glass compositions having a content of iron oxide of 50 to 200 parts per million (ppm) as often occurs in the case of TiO2 containing borosilicate glasses are prone to discoloration during the melting process. The contents of iron oxide are common and are caused by impurities of the main components of the glass with this substance. The discoloration is due to the formation of iron(II) titanate, thus FeTiO3 or ilmenite, which is—formally speaking—a mixed compound of titanium dioxide TiO2 and iron(II) oxide, FeO. For the formation of ilmenite it is necessary that the iron which is present in the glass melt is present in its divalent oxidation state. On the other hand, the formation of ite is inhibited, when large portions of the iron are present in the oxidation state +III, as in the case of Fe2O3.
This can be achieved with the addition of oxidizing substances to the glass melt which cause successive release of elementary oxygen or other oxidizing gases in the glass melt and thus provide a more or less constant concentration of the oxidizing gas in the glass melt. The constant level of the oxidizing gases in turn results in that the metals which are present in the glass melt, also including iron, remain in their higher oxidation states or after all are converted into them during the melting and refining process. In oxidized form these compounds normally only cause mild to no discoloration of the glass. Also during and after solidification such oxidized compounds are not reduced.
For achieving a permanently high level of O2 in the glass melt, in practice preferably a mixture of arsenic and antimony oxides or different oxides of arsenic or antimony separately from each other are used.
Such mixtures gradually release oxygen gas, wherein antimony oxide has a high peak of O2 release at about 1150 degrees Celsius (° C.), whereas the peak of O2 release of arsenic oxide reaches temperatures of up to 1250° C. or higher. Thus, these oxides work at temperatures which allow their use as refining agents in a lot of glass melts.
A great disadvantage of these refining systems of arsenic and antimony oxides is the high toxicity and environmental harmfulness of the arsenic and antimony compounds, for which reason their use is already limited today by legal restrictions which in future may become more strict. Therefore, in glass industry there is a demand for suitable substitutes of these effective refining agents.
An option which is often chosen is the use of sulfates, tin oxide, halides or a combination of these substances. But these substances normally result in a much lower oxidation effect in the glass melts during the refining process which in turn leads to brown coloration of the glasses by the formation of ilmenite.
At higher process temperatures halides are highly volatile compounds which in turn results in quick depletion of the glass melts from these compounds. This in turn makes it necessary to overdose the halide refining agents. This overdose in turn results in higher emissions of hydrohalogen acids. On the one hand, this is not desirable in view of environmental compatibility of the process. On the other hand, the increased release of acids results in accelerated wear of the production facilities and thus increases the process costs in the long term.
Also nitrates, such as for example potassium nitrate, which are optionally used as substitutes of refining agents, have the tendency of releasing the oxidizing gases generated during the decomposition, especially the oxygen, too early, i.e. at temperatures which are too low into the glass melt. For this reason, they are also poorly suitable for use in a refining process at temperatures of above 1000° C. Preferably, nitrates are used in combination with arsenic(III) oxide or antimony(III) oxide which are thereby converted in the glass melt into As(V) oxide or antimony(V) oxide. As such they release oxygen during the refining process.
A further conceivable possibility for generating an oxidizing milieu in the glass melt is the direct bubbling of oxygen into the melt aggregates. However, such a method is only suitable for such processes in which large process receptacles are used. But with such methods finely divided oxygen in the glass melt cannot be achieved with the technical possibilities available till today in contrast to the use of refining agents.